1. Field of the Invention
The present invention relates to axial thrust ball bearings in which at least one of the bearing races has two annular raised areas providing for rolling contact for the bearing balls, and to traction roller transmissions including such ball bearings.
2. Description of the Prior Art
A ball bearing consists of two races which form therebetween an annular cavity receiving the bearing balls. Upon rotation of the two races relative to each other the balls roll along the races while transmitting any load between the races with only rolling friction. To keep the friction low, the curvature of the races in an intersecting plane which receives the axis of the races is slightly larger than the curvature of the balls so as to avoid spin. The width of such a circular contact area depends on the curvature difference between the balls and the races (conformity). Conformity is defined as the ratio of the ball radius to the radius of curvature of the bearing cavity and is in the range of 0.95 to 0.97 for most bearings. With close conformity between balls and races the contact area, which, on a flat surface, would be a small circular area, becomes an elongated elliptical area. Since this elliptical area is however also curved along its main axis the internal frictional losses are quite appreciable.
If, on the other hand, there is no close conformity between balls and races there is little friction but the surface pressure in the contact circle area is greater and bearing life may be shorter. The balls are in contact with the races only along narrow circular lines along which the races are heavily loaded while other portions of the races bear little or no load.
In order to distribute the load over a larger area of the races and thereby permit larger loads for a given bearing size, at least one of the bearing races is so curved that it has two circular contact areas with the balls. This is obtained for example by providing on such race different surface areas whose centers of curvature are spaced from each other. The balls of such ball bearings contact the respective race along two lines of contact which reduces the contact pressures to one half and, consequently increases bearing life or permits higher bearing loads. There are bearings with three or four annular contact areas between the races and the bearing balls, as for example disclosed in U.S. Pat. Nos. 832,427 and 3,586,396. These bearings have high radial load capacity with low friction but axial load may cause spin on one or two of the annular contact areas which would result in rapid wear of the bearing.
In traction roller transmissions of the type described in U.S. Ser. No. 350,187 the traction rollers are exposed to high axial forces but the bearings supporting the rollers need to be relatively small since they are to be accommodated within the traction rollers. Bearings capable of supporting large axial loads are therefore required and bearings with more than only one pair of annular contact areas would be desirable.